This invention relates to electrical storage cells, and, more particularly, to a planar sodium-sulfur storage cell having a high energy storage density.
Rechargeable cells are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A number of such cells are typically connected together electrically to form a battery having specific voltage or current delivery capability. Familiar examples of the rechargeable cell are the lead-acid cell used in automobiles and the nickel-cadmium cell used in portable electronic devices such as cameras. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
Yet another type of cell is the sodium-sulfur cell, which has been under development for about 20 years for use in a variety of terrestrial applications such as nonpolluting electric vehicles. The sodium-sulfur cell has the particular advantage that its storage capacity per unit weight of cell is several times the storage capacity of the nickel-hydrogen cell. The sodium-sulfur cell therefore is an attractive candidate for use in spacecraft applications.
The most common type of construction for a sodium sulfur cell includes a cylindrical metal outer housing which serves as a positive terminal and a cylindrical shell of an alumina-based ceramic within the outer housing. Sodium is placed into a first or inner chamber formed within the alumina shell, and sulfur is placed into a second chamber formed between the alumina shell and the outer housing. The cell is heated to a temperature of about 350 C., at which temperature both the sodium and the sulfur are molten. The liquid sodium acts as the anode of the cell, the liquid sulfur acts as the cathode, and the solid ceramic acts as the electrolyte. Electrical energy is released when sodium ions diffuse through the ceramic into the sulfur, thereby forming sodium polysulfides. Electrical energy can be stored when the process is reversed, with an applied voltage causing the sodium polysulfides to decompose to yield sodium and sulfur, and the sodium ions diffuse through the ceramic electrolyte back into the first chamber.
Planar sodium-sulfur cells have also been developed. The term "planar" is used in this context to mean that the geometry of the cell is larger in lateral extent relative to its height, as compared with the cylindrical cell, and that the anode and cathode are generally planar. The planar design has the advantage that the active area of electrolyte is relatively larger per unit weight of cell than for a cylindrical design. Planar sodium-sulfur cells are disclosed in U.S. Pat. Nos. 3,765,945 and 3,783,024, for example.
The sodium sulfur cell is under consideration for many applications requiring a high capacity of electrical energy storage, such as electrically powered automobiles. It has not as yet found widespread use because of the state of development of such electrically powered vehicles, and because of engineering problems associated with the operation of the cell at elevated temperatures in a vehicle.
The sodium-sulfur cell is also a candidate for use in energy storage for spacecraft such as communications satellites. A satellite orbiting the earth is exposed to intense sunlight and then plunged into shadow in a periodic manner. In some satellites, electrical energy to power the systems on board the satellite is created by solar cells that function when the satellite is in sunlight, and a portion of the electrical energy so generated is stored in electrical storage cells. The stored energy is then available for use when the satellite is in the earth's shadow or for peak power demands, by discharging the cells.
Neither cylindrical nor planar sodium-sulfur cells have been extensively used in spacecraft and terrestrial applications as yet because of a number of problems, including their reliability and failure mechanisms. Many existing designs have promise in such applications, but have not been demonstrated to reliably display sufficient operating lives, and upon failure would render inoperable a battery incorporating the cell if mechanical bypass protection were not provided in the battery design.
There is a need for an improved sodium-sulfur storage cell having a high energy storage density and a favorable failure mode that does not render the entire battery inoperable. The present invention fulfills this need, and further provides related advantages.